Rainfall deficiencies intensify

Widespread
dry conditions throughout eastern Australia in January intensified
rainfall deficiencies,
the Bureau of Meteorology announced today. Furthermore, the situation
was exacerbated by continuing well above average temperatures. One
area that did experience relief was around the Gulf of Carpentaria,
particularly in the NT, where heavy monsoonal rain ended periods of
rainfall deficiencies.

10-month rainfall deficiencies

For the ten-month period from April to January,
serious to severe rainfall deficiencies covered the vast majority of
Queensland, NSW and Victoria, more than half of South Australia, much
of the southern NT, significant areas in southwest WA and the far
east of Tasmania. A large area through central Queensland and northern
NSW has had record low rainfall for the April to January period,
with records dating from 1900.

14-month rainfall deficiencies

For the fourteen-month period from December 2001
to January 2003, some additional areas between Cooktown and
Townsville in far north Queensland, and in the southeast of the State
from Rockhampton to the NSW border have also experienced serious to
severe rainfall deficiencies.

The period to the end of March remains the most likely time for a
significant change in Australia’s rainfall patterns. This relates to
the end of the current El Nino event,
which is now showing clear signs of decay. These changed rainfall patterns,
which usually mark the beginning of the end of drought conditions, can
take various forms ranging from a succession of moderately wetter than
average months, to a widespread deluge that heralds several months of
significantly above average rainfall.

It is not possible for the Bureau to say just how or when this current
period of rainfall deficiencies will end, but because they are so
widespread, the breakdown is unlikely to be uniform in either space or time.
Furthermore, different industries are impacted by different lengths of
periods of rainfall deficiencies and for example, it may take water supply
storages longer to recover from a period of rainfall deficiency than a
farming or grazing activity.

Comparisons with other dry spells

A frequently asked question is "how does the current drought
compare with other notable ones from the past?" It should be noted
that the Commonwealth Department of Agriculture Fisheries and
Forestry - Australia (AFFA) takes the lead on drought impact issues at
the national level, and official drought declarations are the
responsibility of the State and Territory Governments. The Bureau of
Meteorology advises AFFA on all aspects of climate conditions.

The Bureau’s National Climate Centre monitors Australian rainfall
patterns on a range of time-scales from days to years, including analyses
of rainfall deficiency that form the basis for statements like those
above. To reiterate, rainfall is said to be deficient if it is within
the lowest 10% of historical totals (decile range 1 or below the 10th
percentile) for the period in question (3 months or longer).

The National Climate Centre has analysed the historical rainfall
record back to 1900 looking at both the extent of areas below the 10th
percentile, as well as the mean Australian percentile ranking. The
10-month period from April to January is compared to all ten-month
periods of previous Australian droughts with the results tabulated below.
Note that for long droughts with multiple overlapping 10-month periods,
only the most intense period is shown in the table.

Rank

10-month period

% of Australia
below the 10th
percentile

Rank

10-month period

Mean Australian
percentile value

1.

Nov 1901 - Aug 1902

61.2

1.

May 1982 - Feb 1983

16.2

2.

May 1982 - Feb 1983

60.3

2.

March - Dec 1940

17.5

3.

Apr 2002 - Jan 2003

53.8

3.

March - Dec 1994

18.3

4.

Feb - Nov 1940

51.0

4.

Feb - Nov 1902

18.9

5.

July 1951 - Apr 1952

45.2

5.

Apr 2002 - Jan 2003

19.0

6.

May 1928 - Feb 1929

45.0

6.

Apr 1946 - Jan 1947

19.2

7.

Apr 1946 - Jan 1947

45.0

7.

March - Dec 1938

19.7

8.

Oct 1964 - July 1965

43.6

8.

July 1951 - Apr 1952

20.9

9.

March - Dec 1994

42.1

9.

Apr 1928 - Jan 1929

22.2

10.

March - Dec 1918

39.9

10.

Nov 1969 - Aug 1970

22.5

So in an Australia-wide sense, the current drought is among the
worst on record, as far as relatively short to medium-duration
events are concerned, being remarkable for both its spatial extent
of rainfall deficiencies, and average level of "dryness".
Even outside of the rainfall deficient areas, the past ten months
were mainly drier than average with very few regions experiencing
average or above average totals. The main exception is around the
southern Gulf of Carpentaria where recent very heavy rain has produced
the wettest April to January period on record!

Ultimately it will only be at the conclusion of the current event
that its true historical context can be calculated.